This application claims benefit of priority under 35 USC xc2xa7119 to Japanese Patent Application No. P2000-282236, filed on Sep. 18, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electrophotographic device capable of printing high quality images on a receptor sheet (or a copy sheet) with high speed.
2. Description of the Related Art
Recently, there is a strong demand for printing high quality color images with high speed following the progress of color printer technology. Ink-jet technology and sublimation type printer technology can provide high quality color images, but they print at a low speed. On the contrary, electrophotography technology can print a color image with high speed, but provides a low quality color image.
There is a Tandem method to realize high speed printing, as one of the color image formation methods using electrophotography technology. The Tandem method includes four image forming units arranged in parallel (each unit made up of a photosensitive drum, a charging unit, an exposure unit, and a development unit) corresponding to yellow, magenta, cyan, and black. In the Tandem method, each color image is transferred onto an image receptor sheet (or a printing sheet) in turn after each color development has been completed. Although this method can print color images on the printing sheet with high speed, it is difficult to obtain a high quality color image on the printing sheet because it is difficult to position each color image on the printing sheet accurately.
On the contrary, there is a well known dry-toner-type electrophotographic method of forming each of four color toner images onto a photosensitive layer placed on the outer surface of a photosensitive drum by four image forming units corresponding to each of the four colors, developing these toner images superimposed on the photosensitive layer, and then transferring those images onto a printing sheet at once (hereinafter, referred to as xe2x80x9cImage on image development methodxe2x80x9d). In this method, during the rotation of the photosensitive drum, four color toner images can be formed on the photosensitive layer with high speed like the speed of the Tandem method. In addition, the method has another feature to keep the positioning of each toner color precise. Therefore this method can also provide a high quality color image.
However, this conventional dry-toner-type electrophotographic method has essential problems that make it difficult to obtain high quality color images. For example, the quality of the color print image is reduced because toner images are scattered while the plural color toners are superimposed on the photosensitive layer when the image on image development method is used.
This is the phenomenon in which a previously developed toner image on the photosensitive layer is scattered during the process of forming a subsequent color image. The reason is as follows:
After developing the previous toner image, when the photosensitive layer is charged and the selective-exposure is performed to the image forming region for the subsequent toner, a large-voltage potential difference is generated at the boundary area between an exposed area and an unexposed area which are adjacent to each other. This generates a large electric field along a lateral direction. At this time, when the previously developed toner is present in the unexposed area adjacent to the exposed area, the previously developed toner is scattered by a strong electrostatic force in the lateral direction caused by the electric field.
As described above, the conventional dry-toner-type color electrophotographic devices using toner powders have the drawback that it is difficult to obtain a desired high-quality print image by applying the image on image development method because the previously developed toner is scattered during the development process for the subsequent toner image.
Accordingly, an object of the present invention is, with due consideration to the drawbacks of the conventional technique, to provide an electrophotographic device capable of outputting a high quality color image with high speed even if the image on image development method is used.
In accordance with an embodiment of the present invention, an electrophotographic device comprises a photosensitive layer formed on a photosensitive drum, charging units, exposure units, development units, a transfer section, detectors, and a control section which controls the operations of these configuration elements. The control section controls operations of at least one or more of the charging units, the exposure units, and the development units in order to obtain forces satisfying a relationship designated by a following inequality, in which these forces are applied to a previously developed toners close to selectively-exposed subsequent toners where no previously developed toners are present, wherein the previously developed toner area is not exposed during this selective exposure process performed after the completion of the charging process by the charging unit after the completion of the development for the previous toner by the previous development unit,             q      ⁢              xe2x80x83            ⁢                        Δ          ⁢                      xe2x80x83                    ⁢          V                          10                      -            4                                ≤                            R                      12            ⁢                          D              2                                      ⁡                  [                                                                      3                  ⁢                  kT                                4                            ⁢                                                (                                                                                    ϵ                        t                                            -                                              ϵ                        m                                                                                                            ϵ                        t                                            +                                              ϵ                        m                                                                              )                                2                                      +                                                            3                  ⁢                                      hV                    e                                                                    16                  ⁢                                      2                                                              ⁢                              xe2x80x83                            ⁢                                                                    (                                                                  n                        t                        2                                            -                                              n                        m                        2                                                              )                                    2                                                                      (                                                                  n                        t                        2                                            +                                              n                        m                        2                                                              )                                                        3                    2                                                                                ]                    -                        1                      4            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢                          ϵ              m                        ⁢                          ϵ              0                                      ⁢                  xe2x80x83                ⁢                              q            2                                R            2                                ,
where q is a toner charge per previously developed toner, xcex94V is a voltage difference between a surface voltage Vo of the previously developed toner after the completion of the subsequent charging process and a surface voltage VL at a selectively-exposed area, R is a radius of a previously developed toner, D is a distance between previously developed toners (a surface distance), xcex5t and nt are a relative dielectric constant and a refractive index of the previously developed toner, respectively, xcex5m and nm are a relative dielectric constant and a refractive index of a medium in which the previously developed toners are present, respectively, k is Boltzmann""s constant, xe2x80x9cTxe2x80x9d is absolute temperature, xe2x80x9cxcex50xe2x80x9d is a dielectric constant of a vacuum, xe2x80x9chxe2x80x9d is Planck""s constant, and xe2x80x9cVexe2x80x9d is a toner-absorption frequency.